The invention relates to a method of tensioning, but more particularly, the invention relates to a method that may be used for a synchronous belt drive system. While the tensioner of the invention may be used in different belt drive system applications, it is particularly useful in solving belt tension problems associated with camshaft belt drive systems for automotive applications.
Synchronous belt systems, like most mechanical systems, include components with inertias and elasticities and, as a result, such belt systems are capable of vibration and can be characterized from a vibration point of view, by natural frequencies. The most important type of system vibration that relates to the invention is angular vibration where angular vibration is for example, the oscillation of pulleys about their axes of rotation superimposed over relatively constant angular velocities of those pulleys of the system.
Vibrations can be detrimental to the system operation. The worst condition may occur at system resonance when the forces associated with pulley motion (i.e., angular vibration) oscillate with a frequency equal to the system natural frequency.
In a camshaft drive system, a synchronous belt is entrained around pulleys that include at least two toothed pulleys. One of the pulleys is a crank pulley and the other is one or more camshaft pulleys that induce cyclic torque variations into the drive system. An idler pulley such as a back side idler pulley is pressed against the belt to effect an installed belt tension.
The one or more camshafts of an automotive belt drive operate over a range of frequencies and induce cyclic torque variations into the drive system. At the frequency of torque variation equal to the system natural frequencies the resonance will occur. Such resonance frequencies will vary with different engine designs. The torque variations contribute to exciting forces that introduce maximum amplitudes of tension variation in the belt at the resonance frequencies. The amplitudes of high tension occur as the camshaft pulleys move dynamically in an opposite direction or at a slower or opposite angular rate than the crankshaft pulley. When the amplitudes of vibration are too high, belt failure may occur by the teeth being sheared off of the synchronous belt at the crank pulley.
It is common practice in many synchronous belt drive systems to minimize the amplitude of maximum resonance tension variations in the system spans by means of a fixed idler pulley. An installation belt tension at a room temperature is chosen to inhibit the amplitude of the variations to prevent belt tooth failure and avoid tooth jump (i.e., ratcheting). If the tension is too low, tooth failure and tooth jump can occur. If installation tension is too high, it can introduce shortened belt life and belt noise at engine operating temperatures. The fixed idler pulley must operate over a range of temperature conditions. There is a change in pulley center distance between a cold engine such as usually occurs during engine start up and a warm or thermally expanded engine such as occurs during normal engine operating temperatures. Thus, the thermal effect is to increase belt tension with engine temperature increases and conversely decrease belt tension with decreases in engine temperature.
The advantage of a fixed idler is that it operates without substantially effecting stiffness of the belt drive system to minimize resonance tension variations due to dynamic effects as introduced by torque variations in the drive. The disadvantage of a fixed idler system is that it is often difficult to consistently set installation belt tensions at a desirable level. System natural frequency and amplitude of vibration at the resonance depends on the belt installation tension. If the tension is too low, resonance will occur which can contribute to dynamic belt failure. If the tensions are too high, noise will result along with belt failure due to over tensioning. A more detailed explanation of the above discussed vibration associated with dynamic characteristics of an automotive camshaft drive system appear in SAE Technical Paper Series No. 880077 by Mizuno, et al.
It might appear to the unskilled that belt tensioners for automotive vehicle accessory such as for example those shown in U.S. Pat. No. 4,299,584 should provide a solution to overcome the variations attributable to thermal and dynamic effects associated with an automotive synchronous drive. Such tensioners are unsatisfactory as being "too soft" because they easily move in an attempt to compensate for belt tension changes as associated with tension variations introduced by cyclic torque variations of the cams. Such accessory drive tensioners adjust to variations in belt tensions by moving an idler pulley against a belt. The movements of the idler pulley are damped to inhibit the amplitude of tensioner vibrations. The damping torque is usually less than 35 percent of the damping force that is effected by the tensioner in maintaining a drive tension. Consequently, the trend for tensioners that accommodate both dynamic and thermal conditions in an automotive camshaft drive are of the hydraulic type such as, for example, shown in U.S. Pat. No. 4,883,446. While such hydraulic tensioners overcome the problem of a fixed idler as is associated with thermal expansion, installation tension, and over tensioning that may cause noise, they also introduce their own problems. Such tensioners are usually very expensive, and they depend on an oil system for pressurizing the actuator which system may fail causing either an over tensioning or under tensioning the belt.